Several low tire detection systems have been described in the literature, including those that monitor tire pressure and profile height. More recently, systems have been described which utilize effective rolling radius calculations to determine when the radius of one of the wheels varies. The generally employed principle of using the effective rolling radius relies on the fact that a wheel with a flat or low pressure tire has an incrementally smaller effective rolling radius than a nominally inflated tire. Often, wheel displacement sensors are used to measure the angular displacement of each wheel. Each of these measurements is related to the effective rolling radius. In this context, the effective rolling radius is defined as the ratio of the true vehicle speed of the center of the wheel divided by the angular velocity or the true forward distance traveled by the center of a wheel divided by the angular displacement measured over this distance.
A previously issued U.S. Pat. No. 5,721,528 by the present inventor, which is incorporated herein by reference, discloses a low tire warning system using wheel speed sensors. This system estimates the vehicle's speed or displacement based on a calculated average of all wheel speeds or displacements, respectively. This previous invention relies on the fact that it is probable that not all tires are in a low inflation condition, and therefore at least some of the tires can be used to estimate the vehicle's speed or displacement. If all tires are equally low, however, no low tire condition may be detected. In addition, the particular tire having the low pressure condition may not be accurately determined.
To prevent the situation where all tires are equally low, periodic warning of a possibility that all tires are low is performed. One method is to measure a period of time which commonly corresponds to the inevitable pressure drop of a tire due to natural factors. For example, a tire generally does not have a perfect seal. Therefore, after a certain elapsed time, the tire will have experienced a pressure drop. Such a system is disclosed in U.S. Pat. No. 5,691,694.
The inventor herein has recognized a disadvantage with the above approach. For example, the correlation between elapsed time and pressure drop is affected by various factors, such as, for example, temperature, atmospheric pressure, vehicle loading, travel distance, and various other conditions. In particular, travel distance has a significant effect on the inevitable tire pressure drop. Thus, a poor correlation between elapsed time and pressure drop is obtained unless other vehicle operation conditions are included.